1. Field of the Invention
The invention relates to underwater sound technology and, in particular, concerns sonar systems with multiple acoustic beams being formed and steered by frequency division.
2. Description of the Related Art
Nearly all under-water vehicles, whether manned or unmanned, are equipped with an ahead-look sonar (ALS). Common applications include obstacle avoidance, mine detection, and rendezvous and docking capabilities. In general, these sonars use electronically beamformed arrays, although some use simpler systems in which multiple, separate directional hydrophones are used to provide multiple preformed beams. The electronically beamformed systems offer improved performance but are substantially more costly because of the complexities of the beamforming circuitry. A typical multi-beam sonar forms about 100 beams over an angular sector of about 150 degrees. The beamforming circuitry for this sonar requires about 100 receiver channel amplifiers to raise the received signal to a level sufficient for digital beamforming of the 100 beams. Because of this receiver amplifier complexity which is proportional to the beam resolution (beamwidth) and the number of beams formed, the multiple physical beam approach is typically limited to a relatively small number of low resolution beams and may be relatively heavy because of the excess of ceramic material in the multiple hydrophones.
Another method of forming multiple beams without an electronic beamformer is the use of an acoustic lens with a multi-element retina of small hydrophones. Although appealing in principle, lens arrays have proven difficult in practice due to issues such as temperature instability and toxicity of fluid materials and shear wave effects in solid lenses. Lens sonars also have problems in terms of the size and weight of the physical beamformer.
Thus, present beamforming techniques have practical cost, size and weight deficiencies. These are particularly important in applications such as small, low cost unmanned under-water vehicles (UUVs) where size and weight are at a premium and the cost of individual subsystems such as sonars preferably needs be kept low.
A frequency scanning technique has been used in radar for many years. Instead of fixed phase shifts between elements, however, the radar implementations use long delay lines between antenna elements or radiating slots in a dispersive delay line. The typical application is to provide vertical scanning of an array where azimuthal scanning is provided by either mechanical rotation or another electronic phase shifting technique.
Hence, there is a need for a sonar system that permits sequential scanning through multiple beams or forming multiple simultaneous acoustic beams. There is need for such sonar system to be implemented in a simple cost and size/weight effective manner.